Aluminum thick film composition(s), electrode(s), semiconductor device(s), and methods of making thereof

ABSTRACT

The present invention is directed to a thick film conductor composition comprised of (a) aluminum-containing powder; (b) one or more glass frit compositions; dispersed in (c) organic medium wherein at least one of said glass frit compositions has a softening point of less than 400° C.

FIELD OF THE INVENTION

The present invention is directed primarily to thick film compositions,electrodes, and semiconductor devices. It is further directed to asilicon semiconductor device. In particular, it pertains to anelectroconductive composition used in the formation of a thick filmelectrode of a solar cell.

BACKGROUND OF THE INVENTION

The present invention can be applied to a broad range of semiconductordevices, although it is especially effective in light-receiving elementssuch as photodiodes and solar cells. The background of the invention isdescribed below with reference to solar cells, as a specific example ofthe prior art.

A conventional solar cell structure with a p-type base has a negativeelectrode that is typically on the front-side or sun side of the celland a positive electrode on the back-side. It is well known thatradiation of an appropriate wavelength falling on a p-n junction of asemiconductor body serves as a source of external energy to generatehole-electron pairs in that body. The potential difference that existsat a p-n junction causes holes and electrons to move across the junctionin opposite directions and thereby give rise to flow of an electriccurrent that is capable of delivering power to an external circuit. Mostsolar cells are in the form of a silicon wafer that has been metallized,i.e., provided with metal contacts which are electrically conductive.

During the formation of the solar cell, an Al paste is generally screenprinted and dried on the back-side of the silicon (Si) wafer. The waferis then fired at a temperature above the melting point of aluminum (Al)to form a Al—Si melt, subsequently, during the cooling phase, aepitaxially grown layer of silicon is formed that is doped with Al. Thislayer is generally called the back surface field (BSF) layer, and helpsto improve the energy conversion efficiency of the solar cell.

Most electric power-generating solar cells currently used are siliconsolar cells. Process flow in mass production is generally aimed atachieving maximum simplification and minimizing manufacturing costs.Electrodes in particular are made by using a method such as screenprinting from a metal paste.

An example of this method of production is described below inconjunction with FIG. 1. FIG. 1 shows a p-type silicon substrate, 10.

In FIG. 1( b), an n-type diffusion layer, 20, of the reverseconductivity type is formed by the thermal diffusion of phosphorus (P)or the like. Phosphorus oxychloride (POCl₃) is commonly used as thegaseous phosphorus diffusion source; other liquid sources are phosphoricacid and the like. In the absence of any particular modification, thediffusion layer, 20, is formed over the entire surface of the siliconsubstrate, 10. This diffusion layer has a sheet resistivity on the orderof several tens of ohms per square (Ω/□), and a thickness of about 0.3to 0.5 μm. The p-n junction is formed where the concentration of thep-type dopant equals the concentration of the n-type dopant,conventional cells that have the p-n junction close to the sun side,have a junction depth between 0.05 and 0.5 um.

After formation of this diffusion layer, excess surface glass is removedfrom the rest of the surfaces by etching by an acid such as hydrofluoricacid. Next, a silicon nitride film, 30, is formed as an anti-reflectioncoating on the n-type diffusion layer, 20, to a thickness of between0.05 and 0.1 um in the manner shown in FIG. 1( d) by a process, such asplasma chemical vapor deposition (CVD).

As shown in FIG. 1( e), a silver paste, 500, for the front electrode isscreen printed then dried over the silicon nitride film, 30. Inaddition, a back-side silver or silver/aluminum paste, 70, and analuminum paste, 60, are then screen printed (or some other applicationmethod) and successively dried on the back-side of the substrate.Normally, the back side silver or silver/aluminum is screen printed ontothe silicon first as two parallel strips or as rectangles ready forsoldering interconnection strings (presoldered copper ribbons), thealuminum is then printed in the bare areas with a slight overlap overthe silver or silver/aluminum. In some cases, the silver orsilver/aluminum is printed after the aluminum has been printed. Firingis then typically carried out in an infrared furnace at a temperaturerange of approximately 700 to 950° C. for a period of from severalseconds to several tens of minutes. The front and back electrodes can befired sequentially or co-fired.

Consequently, as shown in FIG. 1( f), molten aluminum from the pastedissolves the silicon during the firing process and then on coolingdopes the silicon that epitaxially grows from the silicon base, 10,forming a p+ layer, 40, containing a high concentration of aluminumdopant. This layer is generally called the back surface field (BSF)layer, and helps to improve the energy conversion efficiency of thesolar cell.

The aluminum paste is transformed by firing from a dried state, 60, toan aluminum back electrode, 61. Prior art back side aluminum pastestypically utilize aluminum particles of predominantly spherical shapederived from the atomization process where the particles are formedwherein the particle sizes and shapes are not discriminated. Theback-side silver or silver/aluminum paste, 70, is fired at the sametime, becoming a silver or silver/aluminum back electrode, 71. Duringfiring, the boundary between the back side aluminum and the back sidesilver or silver/aluminum assumes an alloy state, and is connectedelectrically as well. The aluminum electrode accounts for most areas ofthe back electrode, owing in part to the need to form a p+ layer, 40.Because soldering to an aluminum electrode is impossible, a silver orsilver/aluminum back electrode is formed over portions of the back side(often as 2-6 mm wide busbars) as an electrode for interconnecting solarcells by means of pre-soldered copper ribbon or the like. In addition,the front electrode-forming silver paste, 500, sinters and penetratesthrough the silicon nitride film, 30, during firing, and is thereby ableto electrically contact the n-type layer, 20. This type of process isgenerally called “firing through.” This fired through state is apparentin layer 501 of FIG. 1( f).

Additionally, while conventional solar cells provide a working design,there is still a need to provide higher efficiency devices. There isalso a need to provide a method of forming such a device at lowertemperatures than the prior art which allows for co-firing over a widerrange of temperatures to provide manufacturers with increasedflexibility and enable compensation of thermal work for thicker andthinner wafer sources. The inventors of the present invention desired toprovide such a higher efficiency device and method for forming such adevice.

Furthermore, there is an on-going effort to provide compositions, whichare Pb-free while at the same time maintaining electrical performanceand other relevant properties of the device. The present inventorsdesired to create novel Al comprising composition(s) and semiconductordevices that simultaneously provide such a Pb-free system while stillmaintaining electrical performance and novel compositions that providesuperior electrical performance. The current invention provides suchcompositions and devices. Furthermore, the composition(s) of the presentinvention lead to reduced bowing in some embodiments of the invention.

SUMMARY OF THE INVENTION

The present invention relates to a thick film conductor composition foruse in forming a p-type electrode. It further relates to the process offorming and use of the composition in semiconductor devices and thesemiconductor device itself.

The present invention is directed to a thick film conductor compositioncomprised of (a) aluminum-containing powder; (b) one or more glass fritcompositions; dispersed in (c) organic medium wherein the softeningpoint of at least one of said glass frit compositions has a softeningpoint of less then 400 degrees C.

The present invention is further directed to a process of forming asolar cell utilizing a silicon substrate having a p-type and an n-typeregion, and a p-n junction, which comprises screen-printing theback-side of said substrate, screen printing the composition as detailedabove, and firing the printed surface at a temperature of 500-990degrees C.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a process flow diagram illustrating the fabrication of asemiconductor device.

Reference numerals shown in FIG. 1 are explained below.

-   -   10: p-type silicon substrate    -   20: n-type diffusion layer    -   30: silicon nitride film, titanium oxide film, or silicon oxide        film    -   40: p+ layer (back surface field, BSF)    -   60: aluminum paste formed on back-side    -   61: aluminum back electrode (obtained by firing back side        aluminum paste)    -   70: silver/aluminum paste formed on back-side    -   71: silver/aluminum back electrode (obtained by firing back-side        silver/aluminum paste)    -   500: silver paste formed on front side    -   501: silver front electrode (formed by firing front side silver        paste)

FIGS. 2 (a)-(d) explain the manufacturing processes for manufacturing asolar cell using the electroconductive paste of the present invention.Reference numerals shown in FIG. 2 are explained below.

-   -   102 Silicon substrate    -   104 Light-receiving surface side electrode    -   106 Paste composition for a first electrode    -   108 Electroconductive paste for a second electrode    -   110 First electrode    -   112 Second electrode

FIG. 3 details bowing as a function of alpha (ppm/K).

FIG. 4 details bowing as a function of softening point.

FIG. 5 presents a dilatometric evaluation of the expansioncharacteristics of Glass D.

FIG. 6 details bowing as a function of the percent of glass frit in a270 micron wafers.

DETAILED DESCRIPTION OF THE INVENTION

The main components of the thick film composition(s) of the presentinvention are an aluminum-containing powder, glass frit, and organicmedium. In one embodiment, the glass frit in the composition is aPb-free glass composition. The composition(s) of the present inventionallow for superior electrical performance as compared to the prior art.The compositions as stated also provide lower bowing than prior artsystems.

Aluminum-Containing Powder

The metallic powder of the present invention is an aluminum-containingpowder. In one embodiment, the aluminum-containing powder comprisesatomized aluminum. The atomized aluminum may be atomized in either airor inert atmosphere. The average particle size distribution of theatomized aluminum powder is in the range of 3 to 50 microns. In oneembodiment, the average particle size distribution of thealuminum-containing powder is in the range of 3 to 20 microns.

The aluminum-containing powder of the present invention may be furtheraccompanied by other metallic powders such as silver-containing powders.

Inorganic Binder(s)-Glass Frit(s)

The aluminum-containing powders described herein above are finelydispersed in an organic medium and are additionally accompanied by oneor more inorganic binders. Specifically, the inorganic binder(s) usefulin the present invention are glass frit(s). The present invention mustcomprise at least one glass frit composition wherein the softening pointof said glass frit composition is less than about 400° C. In oneembodiment, the softening point of at least one of the glass fritcompositions is in the range of 300 to less than 400° C.

The thick film composition may further comprise a glass frit compositionwhich upon firing said glass frit composition undergoesrecrystallization or phase separation and liberates a frit with aseparated phase that has a lower softening point than the originalsoftening point. Thus, the thick film composition comprising such aglass frit upon processing gives lower bowing properties. Typically, theoriginal softening point of the glass frit composition is in the rangeof 325 to 600° C.

In one embodiment, the glass frit of the present invention is a Pb-freeglass frit that has a softening point of less than about 400° C. Anadditional glass frit which may be useful in the present invention is aglass frit composition which upon firing undergoes recrystallization orphase separation and liberates a frit with a separated phase that has alower softening point than the original softening point.

The function of an inorganic binder in an aluminum composition isprimarily provide means to increase the efficiency that the silicon isaccessed by the molten aluminum during the firing process, in additionto this function, the binder will provide some additional cohesion andadhesion properties to the substrate. The need for the inorganic binderin this instance is more important for silicon substrates that havelayers of silica or siliceous glasses as remnants from wafer processing.A further function of the inorganic binder is the affecting of theinfluence of the aluminum layer on the extent of bowing of the finishedcell. The binder can also increase the alloying depth of the aluminuminto the silicon therefore, enhancing or increasing the Al dopantconcentration in the eutectically grown silicon layer.

The chemistry of the glass frit(s) of the present invention areimportant. The glass frit(s) are chosen based on the effectiveness thatit they have on improving the electrical performance of the aluminumthick film paste without compromising other considerations such asenvironmental legislation or public desire to exclude heavy metals ofpotential environmental concern.

The content of the glass frit as an inorganic binder is important inthat it affects the electrical performance of the resultant cell. Thecontent is determined by the glass or inorganic content and is between0.01 percent and 5 percent weight percent based on total thick filmcomposition, with an preferred level for electrical performance andbowing in the range 0.01 weight percent and 2 weight percent dependenton the chemistry of the glass frit.

Some of the glass binders useful in the composition are conventional inthe art. Some examples include borosilicate and aluminosilicate glasses.Examples further include combinations of oxides, such as: B₂O₃, SiO₂,Al₂O₃, CdO, CaO, BaO, ZnO, SiO₂, Na₂O, Li₂O, PbO, and ZrO which may beused independently or in combination to form glass binders. Typicalmetal oxides useful in thick film compositions are conventional in theart and can be, for example, ZnO, MgO, CoO, NiO, FeO, MnO and mixturesthereof. Glass binders that influence the bowing properties are specificin composition.

The conventional glass frits most preferably used are the borosilicatefrits, such as lead borosilicate frit, bismuth, cadmium, barium,calcium, or other alkaline earth borosilicate frits. The preparation ofsuch glass frits is well known and consists, for example, in meltingtogether the constituents of the glass in the form of the oxides of theconstituents and pouring such molten composition into water to form thefrit. The batch ingredients may, of course, be any compounds that willyield the desired oxides under the usual conditions of frit production.For example, boric oxide will be obtained from boric acid, silicondioxide will be produced from flint, barium oxide will be produced frombarium carbonate, etc.

The glass is preferably milled in a ball mill with water or inert lowviscosity, low boiling point organic liquid to reduce the particle sizeof the frit and to obtain a frit of substantially uniform size. It isthen settled in water or said organic liquid to separate fines and thesupernatant fluid containing the fines is removed. Other methods ofclassification may be used as well.

The glasses are prepared by conventional glassmaking techniques, bymixing the desired components in the desired proportions and heating themixture to form a melt. As is well known in the art, heating isconducted to a peak temperature and for a time such that the meltbecomes entirely liquid and homogeneous. The desired glass transitiontemperature is in the range of 325 to 600° C.

It is preferred that at least 85% the inorganic binder particles be0.1-10 μm. The reason for this is that smaller particles having a highsurface area tend to adsorb the organic materials and thus impede cleandecomposition. On the other hand, larger size particles tend to havepoorer sintering characteristics. It is preferred that the weight ratioof inorganic binder to total paste contents be in the range 0.1 to −2.0and more preferably in the range 0.2 to 1.25.

Organic Medium

The inorganic components are typically mixed with an organic medium bymechanical mixing to form viscous compositions called “pastes”, havingsuitable consistency and rheology for printing. A wide variety of inertviscous materials can be used as organic medium. The organic medium mustbe one in which the inorganic components are dispersible with anadequate degree of stability. The rheological properties of the mediummust be such that they lend good application properties to thecomposition, including: stable dispersion of solids, appropriateviscosity and thixotropy for screen printing, appropriate wettability ofthe substrate and the paste solids, a good drying rate, and good firingproperties. The organic vehicle used in the thick film composition ofthe present invention is preferably a nonaqueous inert liquid. Use canbe made of any of various organic vehicles, which may or may not containthickeners, stabilizers and/or other common additives. The organicmedium is typically a solution of polymer(s) in solvent(s).Additionally, a small amount of additives, such as surfactants, may be apart of the organic medium. The most frequently used polymer for thispurpose is ethyl cellulose. Other examples of polymers includeethylhydroxyethyl cellulose, wood rosin, mixtures of ethyl cellulose andphenolic resins, polymethacrylates of lower alcohols, and monobutylether of ethylene glycol monoacetate can also be used. The most widelyused solvents found in thick film compositions are ester alcohols andterpenes such as alpha- or beta-terpineol or mixtures thereof with othersolvents such as kerosene, dibutylphthalate, butyl carbitol, butylcarbitol acetate, hexylene glycol and high boiling alcohols and alcoholesters. In addition, volatile liquids for promoting rapid hardeningafter application on the substrate can be included in the vehicle.Various combinations of these and other solvents are formulated toobtain the viscosity and volatility requirements desired.

The polymer present in the organic medium is in the range of 0 weightpercent to 11 weight percent of the total composition. The thick filmcomposition of the present invention may be adjusted to a predetermined,screen-printable viscosity with the organic polymer-containing medium.

The ratio of organic medium in the thick film composition to theinorganic components in the dispersion is dependent on the method ofapplying the paste and the kind of organic medium used, and it can vary.Usually, the dispersion will contain 40-95 weight percent of inorganiccomponents and 5-60 weight percent of organic medium (vehicle) in orderto obtain good wetting.

Within the claims, the addition of polymers or organic species orinorganic species that provide exothermic chemical reactions between280° C. and 900° C. are found to be beneficial to the overallperformance of the system where these species are not regarded as apoison towards the semiconductor properties of the doped silicon system.

The electroconductive paste of the present invention is typicallyconveniently manufactured by power mixing, a dispersion technique thatis equivalent to the traditional roll milling, roll milling or othermixing technique can also be used. The electroconductive paste of thepresent invention is preferably spread on the desired part of the backface of a solar cell by screen printing; in spreading by such a method,it is preferable to have a viscosity in a prescribed range. Otherapplication methods can be used such as silicone pad printing. Theviscosity of the electroconductive paste of the present invention ispreferably 20-200 PaS when it is measured at a spindle speed of 10 rpmand 25° C. by a utility cup using a Brookfield HBT viscometer and #14spindle.

The Ag/Al or Ag film can be cofired with Al paste of the presentinvention at the same time in a process called cofiring. Next, anexample in which a solar cell is prepared using the electroconductivepaste (aluminum electroconductive paste) of the present invention isexplained, referring to the figure (FIG. 2).

First, a Si substrate 102 is prepared. On the light-receiving side face(surface) of the Si substrate, normally with the p-n junction close tothe surface, electrodes (for example, electrodes mainly composed of Ag)104 are installed (FIG. 2( a)). On the back face of the substrate, a Agor Ag/Al electroconductive paste (although it is not particularlylimited as long as it is used for a solar cell, for example, PV202 orPV502 or PV583 or PV581 (commercially available from E.I. du Pont deNemours and Company)) is spread to form either bus bars or tabs toenable interconnection with other cells set in parallel electricalconfiguration. On the back face of the substrate, the novel aluminumpastes of the present invention are used as a back face (or p-typecontact) electrode for a solar cell, 106 are spread by screen printingusing the pattern that enable slight overlap with the conductive Ag orAg/Al paste referred to above, etc, then dried (FIG. 2( b)). The dryingtemperature of each paste is preferably 150° C. or lower in a staticdrier for 20 minutes or 7 minutes in a belt drier with a temperatureabove 200 C for 3 minutes (DEK drier model 1209 settings: lamp settings9 and speed 3. Also, the aluminum paste preferably has a dried filmthickness of 15-60 μm, and the thickness of the silver/aluminumelectroconductive paste of the present invention is preferably 15-30 μm.Additionally, the overlapped part of the aluminum paste and thesilver/aluminum electroconductive paste is preferably about 0.5-2.5 mm.

Next, the substrate obtained is fired at a temperature of 700-1000° C.for about 3 seconds-15 min, for instance, so that the desired solar cellis obtained (FIG. 2( d)). An electrode is formed from the composition(s)of the present invention wherein said composition has been fired toremove the organic medium and sinter the glass frit.

The solar cell obtained using the electroconductive paste of the presentinvention, as shown in FIG. 2( d), has electrodes 104 on thelight-receiving face (surface) of the substrate (for example, Sisubstrate) 102, Al electrodes 110 mainly composed of Al andsilver/aluminum electrodes 112 mainly composed of Ag and Al, on the backface.

The present invention will be discussed in further detail by givingpractical examples. The scope of the present invention, however, is notlimited in any way by these practical examples.

EXAMPLES

The examples cited here are based on firing said example pastes onwafers that have silicon nitride anti-reflection coating and areconventional cell designs with a front side n-type contact thick filmsilver paste. The performance of the paste is defined in terms of theelectrical properties and additionally the bowing of the cell afterfiring (defined as the deflection of the fired cell at room temperatureand the distance traveled at the center of the wafer to achieve a flatcell).

(1) Aluminum Paste with Glass Frit

Mixtures of aluminum powders with glass frits are described here. Therelative glass content with respect to the aluminum powder content andparticle size affects the electrical properties and the extent of bowingof thinner cells.

In this example, we cite 4 glass compositions, three of which are leadborosilicate compositions and one is a non-lead containing glass. Thesystems differ in that A and B are glass systems that will soften andfreeze, C and D soften but during cooling will crystallize, glass C willremain liquid to temperatures below 350° C.

TABLE 1 Glass compositions cited Weight % as oxides A B C D SiO2 5.406.00 3.50 32.72 Al2O3 4.10 PbO 78.10 80.50 42.40 ZrO2 2.90 B2O3 12.4012.00 3.60 2.90 ZnO 1.50 2.91 MgO 1.17 TiO2 5.23 Na2O 3.10 Li2O 0.87Bi2O3 50.50 48.20 α 9.2 10.5 18.6 12.9 Dilotometric 400 365 404 460Softening point (° C.) α = Temperature coefficient of expansion ppm/KThese glasses are mixed in as in the manner of the art of makingaluminum pastes, into the product L20261 from Silberline (UK) Ltd whichcontains an aluminum powder. The addition by weight of frits A, B, C andD were varied between 0.25% and 2.5% based on 74% weight of aluminumpowder and then printed on 270 micron thick wafers of 125 mm squaremulticrystalline silicon pre-processed to the point where the next stepis printing and firing. The wafers were converted to cells by firing ina Centrotherm 4 zone furnace with zone temperatures defined as zone1=450° C., zone 2=520° C., zone 3=575° C. and the final zone set at 925°C. or 950° C. with a belt speed of 2500 mm/min. The electricalperformance and the measurement of the bow was undertaken, themeasurement of the efficiency is shown in Table 2 and Table 3 and thefill factor (FF) is shown in Table 4 and Table 5 and the bow is shown inTable 6. We should note that from work reported by Young et al (PVSECconference New Orleans), there is a relationship between the electricalperformance and the weight deposit (or thickness) where there is a pointwhere the electrical performance declines seriously if the weight isbelow this value, for this reason, the thickness of the layer isreported in Table 7 where the pastes are printed at thickness above thisso called saturation value.

TABLE 2 Efficiency (%) at peak temperature of 925 C. N-type % % %conductor Organic Al Frit A B C D PV147 26.0 74 0 13.09 13.09 13.0913.09 PV147 25.7 74 0.3 12.97 12.94 13.26 13.40 PV147 25.5 74 0.5 13.3113.44 13.50 13.22 PV147 25.0 74 1 13.14 13.27 13.28 13.10 PV147 24.5 741.5 13.35 13.18 13.21 13.45 PV147 23.5 74 2.5 13.01 13.46 13.11 12.37

TABLE 3 Efficiency (%) at peak temperature of 950 C. N-type % % %conductor Organic Al Frit A B C D PV147 26.0 74 0 12.88 12.88 12.8812.88 PV147 25.7 74 0.3 13.25 12.85 12.87 13.28 PV147 25.5 74 0.5 13.2913.10 13.22 13.12 PV147 25.0 74 1 13.30 13.10 13.35 13.40 PV147 24.5 741.5 13.33 13.25 13.19 13.22 PV147 23.5 74 2.5 12.80 13.08 13.04 13.07

TABLE 4 Fill Factor (%) at peak temperature of 925 C. N-type % % %conductor Organic Al frit A B C D PV147 26 74 0 70.76 70.76 70.76 70.76PV147 25.7 74 0.3 69.98 71.01 72.38 72.32 PV147 25.5 74 0.5 71.26 71.8872.23 72.85 PV147 25 74 1 70.95 72.34 72.30 71.68 PV147 24.5 74 1.572.43 72.85 71.93 72.35 PV147 23.5 74 2.5 71.16 73.07 72.07 68.29

TABLE 5 Fill Factor (%) at peak temperature of 950 C. N-type % % %conductor Organic Al frit A B C D PV147 26 74 0 70.20 70.20 70.20 70.20PV147 25.7 74 0.3 71.23 70.80 70.27 72.06 PV147 25.5 74 0.5 71.22 71.5371.77 71.52 PV147 25 74 1 70.94 71.65 72.26 72.56 PV147 24.5 74 1.572.17 72.87 71.53 72.49 PV147 23.5 74 2.5 70.50 72.41 71.37 72.34

TABLE 6 Bowing in microns on 270 um thick wafers N-type % % % conductorOrganic Al frit A B C D PV147 26 74 0 655 655 655 655 PV147 25.7 74 0.3551 530 499 538 PV147 25.5 74 0.5 570 610 528 556 PV147 25 74 1 772 818637 565 PV147 24.5 74 1.5 852 850 733 460 PV147 23.5 74 2.5 1053 1031957 388

TABLE 7 Fired print thickness (in microns) of Al pastes N-type % %conductor Organic Al A B C D PV147 26 74 0 32.0 32.0 32.0 32.0 PV14725.7 74 0.3 32.4 34.6 29.6 32.2 PV147 25.5 74 0.5 36.0 32.2 31.8 34.4PV147 25 74 1 33.6 32.0 29.4 32.8 PV147 24.5 74 1.5 32.2 43.4 34.8 33.0PV147 23.5 74 2.5 35.4 56.8 34.4 31.8

The examples cited show that the addition of the glass frits A, B, C andD to the aluminum powder alone provides better electrical performance.The performance is a function of frit content, chemistry and the firingtemperature.

Based on a bi-metallic strip model, it is expected that materials thathave an increasing difference of temperature coefficient of expansionfrom the base substrate would expect to have larger bow and that anincreased freezing point materials would also contribute to largerbowing. The bi-metallic strip model would predict that the bowing wouldbecome greater for increasing addition of frit to the system. Theequation for the deflection of a bi-metallic strip is given by

$\delta = \frac{3\left( {\alpha_{b} - \alpha_{a}} \right)\left( {T_{f} - T} \right)\left( {t_{b} + t_{a}} \right)d^{2}}{4{t_{b}^{2}\left( {4 + {6{t_{a}/t_{b}}} + {4\left( {t_{a}/t_{b}} \right)^{2}} + {\left( {E_{a}/E_{b}} \right)\left( {t_{a}/t_{b}} \right)^{3}} + {\left( {E_{b}/E_{a}} \right)\left( {t_{b}/t_{a}} \right)}} \right)}}$

where δ is the deflection (m), t_(a) is the thickness of the top layer(m), t_(b) is the thickness of the bottom layer (m), T_(f) is thefreezing temperature (° C.), T is the measuring temperature (° C.),α_(a) is the TCE for top component (10⁻⁶ K⁻¹), α_(b) is the TCE for thebottom component (10⁻⁶ K⁻¹ 1), E_(a) is the elastic modulus for the topcomponent (Pa), E_(b) is the elastic modulus for the bottom component(Pa) and d is the width of the smaller component (m).

TABLE 8 Bow data (in microns) for 270 um thick wafers for glass A, B, Cand D with temperature coefficient of expansion data (in ppm/K) andsoftening data data ° C. (SP) as a function of % glass content FritAlpha SP 0% 0.3% 0.5% 1% 1.5% 2.5% A 9.2 400 655 551 570 772 852 1053 B10.5 365 655 530 610 818 850 1031 C 18.6 404 655 499 528 637 733 957 D12.9 460 655 538 556 565 460 388So in the examples cited here, we see

-   -   That the extent of bow increases in general for increased frit        in line with the predictions of the bi-metallic strip.    -   That the extent of bow would be expected to increase for glasses        with higher α compared to silicon (2.4 ppm/K). Glasses C and D        do not adhere to the predicted behavior from the bi-metallic        model as the bowing is less.    -   That the extent of bow is decreased when the softening point of        the glass is increased. Glasses C and D do not adhere to this        predicted behavior from the bi-metallic model as the bowing is        less than lower softening point systems.    -   That the extent of bow below 0.5% addition can be less than the        system without glass at all and is a property of the addition        and not the frit chemistry cited in these examples.    -   Glass frits C and D are known to recrystallize (phase        separation) during cooling into a crystalline precipitate within        the glass surrounded by a lower softening or freezing point        phase that the system that was originally added to the system.        FIG. 4 represents the bowing as a function of softening point in        degrees C.    -   Uniquely, the extent of bow of the systems that crystallize        cited here can be lower as the frit content is increased as        shown for glass D. In the case of glass D, α, is negative        between room temperature and 150 degrees C. approximately, as        shown in the dilatometric trace shown in FIG. 5. The ability for        the system to facilitate lower bow than the conventional frits        enables this system to provide very low bow in silicon cells of        thickness of below 225 microns and thus for manufacturers to use        them during post firing handling and module manufacturer with        less tendency to break due to handling difficulties. FIG. 6        demonstrates the bowing performance in 270 micron wafers        (125×125 mm) of each of the identified glass frits.

Manufacture of Solar Cell

The present invention can be applied to a broad range of semiconductordevices, although it is especially effective in light-receiving elementssuch as photodiodes and solar cells. The discussion below describes howa solar cell was formed utilizing the composition(s) of presentinvention.

Using the aluminum electroconductive paste obtained, a solar cell wasformed in the following sequence. Those skilled in the art recognizethat various method of forming solar cells exist and that the thick filmof the present invention may be used in multiple methods.

(1) On the back face of a Si substrate having a silver electrode on thefront surface (for example, PV147 Ag composition commercially availablefrom E.I. du Pont de Nemours and Company) was printed and dried. Typicaldried thickness is in the range of 15 to 25 microns. Then the Ag orAg/Al paste (for example, PV202 is a Ag/Al composition commerciallyavailable from E.I. du Pont de Nemours and Company) was printed anddried as 5-6 mm wide bus bars. Then, an aluminum paste for the back faceelectrode of a solar cell (represents the novel compositions of thepresent invention) was screen-printed at a dried film thickness of 30-60μm providing overlap of the aluminum film with the Ag/Al busbar for 1 mmat both edges to ensure electrical continuity. The aluminum paste wasdried before firing.

(2) The printed wafers were then fired in a furnace with peaktemperature settings of 850 to 965° C. for 3 seconds to 10 minutes,depending on the furnace dimensions and temperature settings. A solarcell was formed after firing.

Test Procedure-Efficiency

The solar cells built according to the method described above wereplaced in a commercial IV tester for measuring efficiencies (IEET Ltd).The lamp in the IV tester simulated the sunlight with a known intensityand radiated the front surface of the cell, the bus bars printed in thefront of the cell were connected to the multiple probes of the IV testerand the electrical signals were transmitted through the probes to thecomputer for calculating efficiencies.

Solar cell wafers were prepared by using a standard frontside contactpaste PV147 Ag conductor (commercially available from E.I. du Pont deNemours and Company).

Samples were printed onto Wafers Supplied by a PV cell manufacturer thatwere processed to the point where the thick film pastes were applied andfired. Processed wafers were then measured for electrical performance.Results indicate that the use of frits A, B, C and D when added to Alpowder has improved the electrical performance versus the unfrittedsystem.

What is claimed is:
 1. A thick film conductor composition consisting of:(a) aluminum-containing powder (b) one or more glass frit compositions,wherein the one or more glass frit compositions are 1 to 2.5 wt % of thethick film conductor composition, dispersed in (c) organic mediumwherein at least one of said glass frit compositions has a softeningpoint of less than 400 degrees C.
 2. The composition of claim 1 whereinone or more of said glass frit compositions is a lead-free glass fritcomposition.
 3. The composition of claim 1 wherein said organic mediumcomprises a polymeric binder and a volatile organic solvent.
 4. Thecomposition of claim 1 wherein upon firing said composition provides anexothermic chemical reaction between 280° C. and 900° C.
 5. A process offorming a solar cell utilizing a silicon substrate having a p-type andan n-type region, and a p-n junction, which comprises screen-printingthe back-side of said substrate, screen printing the composition ofclaim 1, and firing the printed surface at a temperature of 500-990degrees C.
 6. An electrode comprising the composition of claim 1 whereinsaid composition has been fired to remove the organic medium and sinterthe glass frit.
 7. A semiconductor device comprising the electrode ofclaim
 6. 8. A semiconductor device formed by the process of claim
 5. 9.A semiconductor device which utilizes the thick film composition ofclaim
 1. 10. The composition of claim 1 wherein one or more of saidglass frit compositions comprises a glass frit composition that exhibitsnegative temperature coefficient of expansion in the temperature rangeof 20° C. to 200° C.
 11. The composition of claim 1 wherein thesoftening point of at least one of said glass frit compositions is inthe range of 300 degrees C. to less than 400 degrees C.
 12. Thecomposition of claim 3, wherein the polymeric binder comprises ethylcellulose.
 13. The composition of claim 3, wherein the solvent comprisesterpineol.
 14. The composition of claim 1 wherein one or more of saidglass frit compositions comprises lead borosilicate.
 15. The compositionof claim 1, wherein the one or more glass frit compositions compriseoxides selected from the group consisting of B₂O₃, SiO₂, Al₂O₃, CdO,CaO, BaO, ZnO, Na₂O, Li₂O, PbO, ZrO and combinations thereof.